FIG. 1 illustrates a prior art switching power supply. The system of FIG. 1 includes a modulator 10 that generates switching control signals SC1 and SC2 to drive switch circuits 12 and 14, thereby controlling the amount of power delivered to the load 16 through inductors 18 and 20. An output capacitor 22, which includes an equivalent series resistance (ESR), filters the output from the inductors. A voltage mode error amplifier circuit 24 generates an error signal VERR in response to the output voltage VOUT so the modulator can modulate the switch signals to maintain a constant output voltage regardless of the amount of current consumed by the load. The sensed output voltage is combined with an input signal VREF to generate the error signal that is applied to the modulator for closed-loop control of the output. The modulator 10 shown in FIG. 1 is assumed to provide pulse-width modulation (PWM), but other modulation schemes such as pulse frequency modulation (PFM), hysteretic control (ripple regulation), etc., may be used.
The system of FIG. 1 also includes a current sensing circuit 26 to generate a signal VCS that provides a measure of the total combined output current delivered to the load. The current sense signal may be used in numerous ways. For example, it may be used to provide over-current shutdown, it may be used to implement current-mode regulation, or it may be combined with voltage feedback to establish a droop impedance for adaptive voltage positioning (AVP) control schemes.
The system of FIG. 1 is known as a multi-phase switching power supply because the power components including the switches and inductors are repeated to produce multiple output currents that are summed together to provide the total output current. This increases the amount of current available from the power supply. The system of FIG. 1 has two phases, but any suitable number of phases may be used. Multi-phase switching power supplies are commonly used to provide power for microprocessors which require very large currents at voltages that must be regulated to within tight limits to prevent damage to the CPU.
To generate the multiple switching control signals SC1 and SC2 needed for multi-phase switching, the modulation circuit 10 includes multiple comparators 28,30 and driver logic 32. Each comparator receives a ramp signal at one input and the error signal VERR at the other input. The comparator outputs are gated by driver logic 32 so the switching control signals SC1 and SC2 are pulsed in sequence to drive the switches properly, and to provide various features such as current limiting, over/under voltage shutdown, etc. The ramp signals are generated by oscillator 34 at a frequency that is determined by the value of a resistor RT.
A current balance circuit 36 adjusts the individual ramp signals for each phase to prevent thermal or electrical overstress that can occur when one phase delivers more current than the other phases. A typical current balance circuit may operate by comparing the current delivered by each individual phase to a common reference point. In the system of FIG. 1, the individual phase currents may be monitored through sense resistors RS. The modulation of each switching control signal can then be adjusted so that all phases deliver the same amount of current.